The exemplary embodiment relates to systems and methods for detecting image quality defects generated by a rendering device. It finds particular application in connection with a system and method for background noise detection on rendered customer documents and will be described with reference thereto.
Image rendering devices, such as photocopiers, printers, and facsimile machines, render digitally stored images in tangible form by applying marking materials, such as toners or inks, to print media, such as paper. One of the common print defects exhibited in such devices is background noise, which results in the rendered images having small marks that are not present in the original digital image. In electrographic image rendering devices, background noise generally consists of small dots caused by single toner particles or small clusters of toner particles. One source of background noise is “wrong sign toner” (an incorrect charge on a small minority of the toner particles), resulting in the particles being attracted to charged regions of the photoreceptor intended to be blank for that particular color separation. In devices of other non-impact printing technologies, background noise may result from a mist formed from the ink or other marking material which enters the air and subsequently adheres to the printed page. As a consequence, background noise generally lacks a repetitive distribution pattern (giving the appearance of salt and pepper sprinkled on a page), when compared with other image defects, such as bands or streaks. In a uniform region of the image, the dots may appear as a variation in the gray level. In general, “gray” refers to the optical density or area coverage value of any single color separation layer, whether the colorant is black, cyan, magenta, yellow, or some other color. When the level of background noise is low, the dots are often not visually objectionable. However, when more severe, corrective steps may be required to mitigate the problem. Because of the non-regularity of the background noise distribution, this may require adjustments to the device or a service visit for replacement of components of the image rendering device.
In one method for detecting imaging problems caused by an intermediate imaging member, a scanned, printed image of test patches is compared with its original printed image on a pixel by pixel basis. Because of the difficulty of accurately aligning images, the occasional background noise spots are treated as spurious results, below the detection threshold, and thus may not be detected by such a method.
Although background noise may not be perceptible to the naked eye, such image quality defects may indicate that the device is deteriorating, and presage image quality defects that are visible. Accordingly, it would be desirable to detect the image quality defects early, so the image rendering device may be adjusted or repaired.